Microwave attenuator



Sept. 24, 1963 H.`J.,R1BLET MICROWAVE ATTENUATOR Filed Feb. 27. 1958 HENRY J. RIBLET` AT ORNEY IN VEN TOR.

///d////%/9///// //////J-/// ////f/////////////7//////// /K/ United States Patent O 3,105,210 WSRWAVE ATTENUATR Henry Ll. Rihlet, 35 Edmunds Road, Wellesley, Mass. *Filed Feb. 27, 1953, Ser. No. 717,923 4 Claims. (Cl. 333-81) The present invention relates in general to microwave attenuators and more particularly concerns an attenuating directional coupler continuously adjustable to accurately provide a wide range of attenuation. initial insertion loss is minimized and the level of attenuation is virtually independent of frequency.

A method of providing stepwise attenuation in a directional coupler is disclosed by applicant in Patent No. 2,735,069. A `directional coupler is there shown comprising parallel rectangular waveguides exchanging microwave energy through slots in a common side wall. Sliding a plate of material opaque to microwave energy across the slot-s controls the number of slots coupling energy between the guides. Attenuation is increased in stepwise increments by covering more slots. For single frequency applications where very accurate attenuation is not required, satisfactory results are obtained. Although using a lossless opaque plate is frequently advantageous, its position adects the impedance characteristics of the directional coupler such that resonance effects occur at diiferent frequencies and the level of attenuation is a function of frequency as well as the number of slots covered.

The present invention contemplates and has as a primary object the provision of means for selectively attenuating a microwave signal in a directional coupler over a wide continuous range of attenuation, the degree of attenuation being virtually independent of frequency.

According to the invention, main and auxiliary waveguides exchange energy through openings in a common wall therebetween. An attenuating member formed of lossy material projects into the auxiliary waveguide from one end. This member is provided with a leading edge shaped to provide a good impedance match, thereby avoiding resonances and attenuating the incident microwave power through frequency insensitive absorption. The degree of attenuation is related to the volume of lossy material intercepting energy transferred through the openings; hence, markings on the member corresponding to its position in the auxiliary guide may be calibrated in terms of attenuation levels.

According to another feature of the invention, a terminating lossy member absorbs the wave energy at the end of the main waveguide. These and other features, objects and advantages of the invention will be better understood from the following specification when read in connection with the accompanying drawing, in which:

FIG. 1 is a side cross-sectional View in elevation of the invention taken along the line 1--1 of FIG. 2;

FIG. 2 is a top plan cross-sectional View taken substantially along the line 2 2 of FIG. 1 with the exception that the attenuating member of FIG. 1 is removed;

FIG. 3 is a top plan view of the attenuating member; and

FIG. 4 is a fragmentary perspective view of a modification of a detail of the invention.

Referring to the drawings, there is shown a pair of 3 ,105,210 Patented Sept. 24, l 963 waveguides 1 and 2, hereinafter referred to as the auxiliary and main waveguides respectively. The main waveguide 2 receives lmicrowave, energy from a source, not shown, at the end 3, and transmits it to a load at the other end 4, or has a portion of this energy directionally coupled and transmitted to another load at the end 5 of the auxiliary waveguide 1.

lf desired, and it will be seen more fully hereinafter, the energy transmitted to the end 4 may be terminated at a terminating impedance. These waveguides are rectangular in shape and may be provided with a common wide wall 6. The use of a unitary common wide wall per se is not an essential part of the invention. The waveguides 1 and 2 each have the same cross-sectional area and are for-med with continuous narrow side walls and parallel outer side walls. The individual yguides may be secured together by any suitable means, such as, soldering, or by screws as illustrated at 7. The main waveguide 2 is fed through an arcuate channel from the side 9, while the auxiliary waveguide 1 is uniformly straight along the entire length.

The common wide wall 6 may comprise either a single flat plate dividing the two channel members forming the waveguide elements 1 and 2, or two adjacent walls of the separate guides 1 and 2. A series of slots or other suitable openings in the `common wide wall couple energy between the guides. Suitable arrangements and shapes of these slots are more fully described in Patents No. 2,632,809 and No. 2,735,069 of this applicant. Fundamentally, however, the interruption of current ow in the wall surface by the slots creates electromagnetic fields about the slots whereby energy is coupled through the slots from the main waveguide to the auxiliary waveguide. This coupled energy is propagated substantially in one direction, that being the direction Iin which the original source of wave energy was introduced into the main waveguide. The amount of coupled energy is increased by increasing the number of slots, or by increasing the size of the slots. As the size of the slots, however, is constant in this example, variations in the coupled energy are attained by changing the uncovered slots and the volume of attenuating material projecting beyond them.

The slots herein described may be substantially similar to those disclosed in Patent No. 2,735,069. If desired the slots may be tapered in length towards the feed end 3 of the main waveguide as illustrated. Thus, the transverse slots extending normal to the center line of the auxiliary waveguide 1 are tapered at 10 towards a point; similarly, the longitudinally arranged slots 11 are tapered towards the end 12. This tapering of the longitudinal slots 11 causes them to extend only a short distance beyond the transverse slots to which they are normally perpendicular.

The attenuating member 13 is formed of a lossy material such as polyiron. Its main body 14 is wide enough to cover the slots when it is positioned within the auxiliary guide 1. To mini-mize alignment problems, the preferred width is the same as the inner transverse :dimension of the waveguide 1. The main body 14 tapers to a leading end generally indicated at 15, formed with two coplanar probes, 16 and 17 sharply tapered in planes normal and parallel to the common wide Iwall 6. The probes are sufficiently wide at their base to overlap completely the longitudinal slots 11 Aas the attenuating member 13 is slid further into the `auxiliary guide 1. With this arrangement the upper sliding lossy material may be pushed beyond the coupling conguration so that ultimately the attenuation is determined by the amount of ener-gy from the slots which must pass through the increasing volume of lossy material. Also `forming a portion of this rattenuating member 13 is a Calibrating sur-face 19 which extends parallel to the main body 14 and is secured rigidly to it by the post 2G. This calibrating surface 19 is formed preferably of a transparent plastic and hm imprinted thereon, a transverse line 21 adapted to Iact las a reading line for calibrations imprinted upon the yupper surface 22 of the auxiliary waveguide 1: lf desired, only a portion of the attenuating member 13 may be formed of lossy material, provided that lossy material forms at least the probes 16 and 17.

There is also provided ia terminating attenuating member 23 which slides longitudinally into the end 4 of the main waveguide 2. This attenuating terminating member 23 is formed of a lossy material and is tapered both in planes parallel to and normal to the common wide wall 6. Member 23 absorbs energy in waveguide 2 not directionally coupled to waveguide 1.

With reference to FIG. 4,.there is illustrated a modiiication of the attenuating 'member 13 illustrated in FIG. 1. The attenuating member 4@ is also formed of lossy material similar to that of the .attenuating member 13. A center section 41 has a rectangular cross-section. Side members 42 and 43, also of rectangular cross-section, are integrally formed with center section 41 at the center of the inner side walls 44 and 45, respectively. The sections 42 and 43 extend above and below the center section 4i and have a height sufficient to substantially lill waveguide 1. IThe overall width of .the unit between the outer side fwalls of the sections 4?. and 43 is substantially equal to the width of waveguide 1. The individual widths of the sections 41, 42 and 43, `are such as to position the section 41 over substantially all the coupling slots in the common wide wall. At the ends of the attenua-ting member 4d, the Section 41 may project outwardly at 46 beyond the ends of the sections 42 and 43. This attenuating member may be inserted and controlled in a manner similar to attenuating member 13, with the projection 46 extending beyond the end of the waveguide 1. Similarly a calibrating member 19 may be secured to the attenuating member 49. lf desired, the attenuating member 4t? may be controlled and operated in a manner similar to the attenuating Imember disclosed in Patent No. 2,735,069, issued to this applicant on yFebruary 14, 1956. In accordance with that patent, .a handle secured to member 40 might project through a slot in the upper wall of waveguide 1. The techniques illustrated in FIG. 1 are also applicable for adjust-ably positioning member 40 within waveguide 1 'Y at any selected location. When it partially overlaps the slots or other coupling conliguration, the energy passing through the slots is intercepted by only a portion of the attenuating member. In a position fully overlapping the slots, all the directionally coupled energy is intercepted by attenuating member 40 and the volume of -attenuating material through which the energy passes determines the vdegree of attenuation.

`It is to be noted that in the .arrangements heretofore described, the degree of attenuation is continuously con- 1. A microwave attenuator comprising, a pair of parallel rectangular waveguides adapted to convey electromagnetic wave ener-gy substantially in one direction only and having a common Wide wall formed with a plurality of cascaded pairs of spaced mutually perpendicular slots each pair directionally coupling said energy from one of said guides to the other and consisting of a transversely directed slot arranged substantially symmetrically about the center line of said common wall and a longitudinally directed slot having its ycenter along the longitudinal axis of said transversely directed slot, said longitudinally directed slots being arranged alternately adjacent -to opposite side edges of said common wall, and an elongate wave energy attenuating :member having a main body of rectangular cross-section nearly equal to that of said waveguides and slidably positioned within and extendable from the end of one of said waveguides adjacent to said common wide wall, said member being of a lossy material and having a tapered end to absorb increased amounts of wave energy as it is slid :further into said waveguide from said waveguide end.

2. A microwave attenuator comprising, a pair of parallel rectangular waveguides adapted to convey electromagnetic wave energy substantially in one direction only and having a common Wide wall formed with a plurality of pairs of cascaded mutually perpendicular slots each pair directionally coupling sai-d energy from one of said guides to the other and consisting of a transversely directed slot arranged substantially symmetrically about the center line of said common wall and a longitudinally directed slot having its center along lthe longitudinal axis of said transversely directed slot, said longitudinally directed lslots being arranged valternately adjacent to opposite side edges of said common wall, and an elongate wave energy attenuating member slidably extending into one of said waveguides adjacent to said common wide wall through the end of said one waveguide, said member being formed of a lossy material and shaped with a main body portion having a rectangular cross-section substantially the same as said one waveguide .and with its leading end shaped into a pair of co-planar probes each having an axis of symmetry aligned substantially parallel to said longitudinally directed slots, said probes being tapered in planes parallel and normal to said common wide wall.

3. A microwave attenuator comprising, a pair of parallel rectangular waveguides adapted to convey electromagnetic wave energy substantially in one direction only and having a common wide wall formed with a plurality of cascaded pairs of mutually perpendicular slots each pair directionally coupling said energy from one ot said guides to the other and consisting of a transversely directed slot arranged substantially symmetrically about the center line of said common wall `and a longitudinally directed slot in said common wall having its center along the longitudinal axis of said transversely directed slot, said longitudinally directed slots being arranged alternately adjacent to vopposite side edges of said common wall, and an elongate wave energy attenuating member slidably extending into one of said waveguides adjacent to said common wide wall through an end of said one waveguide, said member being formed of a lossy material and shaped with a main body portion having a rectangular crosssection substantially the same as said one waveguide and suiiiciently long to cover said slots with a portion projecting from said one waveguide, and with its leading end shaped into a pair of co-planar probes each having an axis of symmetry substantially parallel to said longitudinally directed slots, said probes being tapered in planes normal and parallel to said common wide wall.

4. In a waveguide directional coupler having a pair of waveguides with a waveguide energy coupling means therebetween, a wave energy attenuating member slidably positioned within one said waveguides through an end of said one waveguide, said member formed of an elongated length of lossy material having a cross sectional area providing a center portion and two outer portions integrally formed therewith on opposite sides thereof, said outer portions extending from the height of the said one waveguide, thereby acting as sliding guides for said member, and said center portion being spaced from and above said coupling means .and extending longitudinally beyond said outer portions, the degree of attenuation imparted by said member being proportional to the volume of said member within said one waveguide.

References Cited in the le of this patent UNITED STATES PATENTS 2,605,400 McLain July 29, 1952 2,735,069 Riblet Feb. 14, 1956 2,812,500 Riolet Nov, 7, 1957 2,820,201 Tomiyasu Ian. 14, 1958 FOREIGN PATENTS 686,062 Great Britain Ian. 14, 1953 

4. IN A WAVEGUIDE DIRECTIONAL COUPLER HAVING A PAIR OF WAVEGUIDES WITH A WAVEGUIDE ENERGY COUPLING MEANS THEREBETWEEN, A WAVE ENERGY ATTENUATING MEMBER SLIDABLY POSITIONED WITHIN ONE OF SAID WAVEGUIDES THROUGH AN END OF SAID ONE WAVEGUIDE, SAID MEMBER FORMED OF AN ELONGATED LENGTH OF LOSSY MATERIAL HAVING A CROSS SECTIONAL AREA PROVIDING A CENTER PORTION AND TWO OUTER PORTIONS INTEGRALLY FORMED THEREWITH ON OPPOSITE SIDES THEREOF, SAID OUTER PORTIONS EXTENDING FROM THE HEIGHT OF THE SAID ONE WAVEGUIDE, THEREBY ACTING AS SLIDING GUIDES FOR SAID MEMBER, AND SAID CENTER PORTION BEING SPACED FROM AND ABOVE SAID COUPLING MEANS AND EXTENDING LONGITUDINALLY BEYOND SAID OUTER PORTIONS, THE DEGREE OF ATTENUATION IMPARTED BY SAID MEMBER BEING PROPORTIONAL TO THE VOLUME OF SAID MEMBER WITHIN SAID ONE WAVEGUIDE. 